1. Field of the Invention
The present invention relates to a device for detecting a combustion condition within each cylinder of an engine, i.e., a misfire detecting device for an ignition system of an internal combustion engine.
2. Description of the Prior Art
A prior art distributor type ignition system incorporating a misfire detecting device, is shown in FIG. 4. The ignition system is generally indicated by "D" and includes an ignition coil 921, a battery 923 connected to a primary winding 922 of the ignition coil 921, a power transistor 924, an ECU (engine control unit) 927 for supplying an ignition signal 925 and a control signal 926 in the form of a rectangular pulse, a distributor 928, a reverse current preventing diode "d", spark plugs 930 each connected at a center electrode side to a side electrode 929 and at an outer electrode to a cylinder side so as to be grounded thereat, a voltage dividing circuit 931, and a combustion condition or misfire detecting circuit 932 for detecting a combustion condition or misfire on the basis of the decay characteristic of a divided voltage.
In the ignition system "D", the pulsed high voltage is set or controlled through control of the time of energisation of the primary winding 922.
However, variations or fluctuations of the voltage of the pulsed high voltage are caused due to variations of the specification of the ignition coil 921, variations of the battery voltage, or variations of the performance characteristics of the ignition coil resulting from variations of the temperature.
For this reason, the pulsed high voltage is set low (by setting the time of energisation to be shorter) so that the spark plug is not caused to fire or discharge by the application of the pulsed high voltage. However, this causes a problem in that the pulsed high voltage cannot go over the gap of the distributor or the accuracy of determination of a misfire based on the voltage waveform of a divided voltage 933 is lowered due to an excessively low detection voltage.
Further, in some kind of ignition system or in some kind of engine it is necessary to make earlier the timing for application of the pulsed high voltage in order to improve the accuracy of detection of a misfire. For example, in the distributor type ignition system "D", energisation of the primary winding 922 needs to be started before completion of firing or spark discharge since the time of application of the pulsed high voltage is limited in relation to the width of the distributor rotor at engine high speed. If energisation of the primary winding 922 is started before completion of firing, the build-up speed of the current becomes faster (as compared with the case the energisation is started after completion of firing). Thus, even if the time of energisation of the primary winding 922 is the same, the pulsed high voltage becomes higher, so that application of the pulsed high voltage for detection of a misfire can easily cause the spark plug to fire.